Long-term potentiation (LTP) and long-term depression (LTD) have become the two most important models for investigating the cellular and molecular mechanisms of learning and memory in the mammalian brain. The proposed experiments are designed to clarify what changes in synaptic structure occur during LTP or LTD. Specific aim 1 is to enhance detection of structural changes in hippocampal slices. The first enhancement will be to prepare hippocampal slices under conditions where changes in synapse structure during LTP or LTD can be evaluated independently of the synaptogenesis that is caused during preparation and incubation of hippocampal slices. Second, stimulated axons will be labeled with colloidal gold particles that are endocytosed from the extracellular space into presynaptic vesicles and endosomes during stimulation of the axons that undergo LTP or LTD. Aim 2 is to determine whether synapse number, synaptic size, or the number of docked presynaptic vesicles are regulated in opposite directions during LTP and LTD at immature hippocampal neurons. Local protein synthesis, indicated by polyribosomes or membrane recycling, indicated by endosomes, will be evaluated to determine whether these processes could mediate the structural changes at synapses that have undergone LTP or LTD. In aim 3 the developmental studies will be extended to mature hippocampal neurons to learn whether changes in synapse number and structure are essential mechanisms of LTP and LTD, or if they only interact with ongoing developmental programs. Despite controversy about the specific roles for the opposing processes of LTP and LTD in shaping memories, it is widely accepted that a clear understanding of their underlying mechanisms will be an important step towards understanding how memories are formed.